CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

7. SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.

I have attempted to show that the geological record is extremely imperfect;
that only a small portion of the globe has been geologically explored with
care; that only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of specimens and of
species, preserved in our museums, is absolutely as nothing compared with
the number of generations which must have passed away even during a single
formation; that, owing to subsidence being almost necessary for the
accumulation of deposits rich in fossil species of many kinds, and thick
enough to outlast future degradation, great intervals of time must have
elapsed between most of our successive formations; that there has probably
been more extinction during the periods of subsidence, and more variation
during the periods of elevation, and during the latter the record will have
been least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is probably
short compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in any
one area and formation; that widely ranging species are those which have
varied most frequently, and have oftenest given rise to new species; that
varieties have at first been local; and lastly, although each species must
have passed through numerous transitional stages, it is probable that the
periods, during which each underwent modification, though many and long as
measured by years, have been short in comparison with the periods during
which each remained in an unchanged condition. These causes, taken
conjointly, will to a large extent explain why--though we do find many
links--we do not find interminable varieties, connecting together all
extinct and existing forms by the finest graduated steps. It should also
be constantly borne in mind that any linking variety between two forms,
which might be found, would be ranked, unless the whole chain could be
perfectly restored, as a new and distinct species; for it is not pretended
that we have any sure criterion by which species and varieties can be
discriminated.